traditionally O2/Voda had the better 900MHz frequencies (for 2G/3G), and T-Mobile and Orange had the less desirable 1.8GHz. Although its all been mixed up now.
Takes me back, both Voda and Cellnet used to have the same take it or leave it fixed Charging Fees, £25 per month for the Airtime Service, 25p per Min (33p in London) rounded up to the nearest Min, Diverted calls to your Voicemail where charged per min, then the same charge to retrieve your messages, then all plus Vat, My Bills, just for the Airtime and Calls were on average between £130 to 180 per month, and that was me using it as a bare minimum, couldnt have run my business without it though.
Still remember the first National Poster Advert from Vodafone, A Bloke in a Rowing Boat, Fishing, with the slogan "How to be in when your Out". also in them days only 2 Prefixes, 0860 for Cellnet and 0836 for Voda, still got my Original 0836 Number today.
Then came along Orange, remember the TV and Radio Advert song 1,2,3,4,...... 5,6,7,8, the first Network to charge by the second, Orange was originally owned and Built by Hutchinson Telecom (Wasn't a bad Network as it goes), who sold it off and then started all over again with Three.
At the Launch of Orange, the other PCN Carrier was 121 owned by Mercury Combinations (C&W), who seemed to have built a Network for London and a few Other Major Cities then kinda left it there, I do remember OfCom giving 121 bollockings every now and then, saying either finish it off or pull out and well have your Licences back, which finally got finished off and became T Mobile.
The original idea of the launch of the 2 PCN Networks (Orange and 121-T Mobile) was never really intended for them to be just another Mobile Network, but more of a Network with Value Added Services with ideas like your PCN Phone would automatically become your indoor phone when at Home or at the Office etc, but due to the same GSM Signalling Format it used, Marketing trends and similar Frequency Bands being released around the World they became just that, another Mobile Network in competition with the GSM Boys.
And then the 5th Network Three, the Only Network to be built on 3G Infrastructure (No 2G unless it was handed over to Other Networks to fill the Gaps). Three was originally launched and Marketed as a Video Calling Network, which really didnt take off, and was soon killed off when peeps discovered they could use Skype over the 3G Channels anyway